This project deals with both the laboratory and clinical aspects of infection caused by the intestinal nematode, Strongyloides stercoralis. The laboratory research involves analysis and characterization of parasite antigens. The clinical studies focus upon the immune response of infected individuals, and factors that influence the immune response. Experimental infections with the parasite in a newly recognized animal host, the jird (Meriones unguiculatus), have been initiated recently. Also, collaborative clinical studies are being carried out in Brazil. The PCR products of internal spacer regions of ribosomal DNA genes of several different species of Strongyloides parasites (S. stercoralis, S. ratti, and S. fulleborni), after treatment with selected restriction enzymes, showed patterns easily differentiated one from the other. Even a dog strain of S. stercoralis from widely different geographic areas of the world showed identical patterns. The cytokine responses in a total of 59 Brazilian patients with different combinations of infection with the retrovirus HTLV-1 and S. stercoralis (positive for both, positive for only one of the pair, or negative for both) have been assayed. Initial focus was on Interferon-gamma (INF-gamma) produced spontaneously and IL-4 produced by mitogenic stimulation of peripheral blood mononuclear cells. Patients infected with HTLV-1 tended to have elevated INF-gamma levels and low levels of IL-4. The low IL-4 responses appear to be correlated with low serum IgE levels and may represent an important contributory factor to complications from strongyloidiasis in patients infected with HTLV-1. Jirds are infected with a dog strain of S. stercoralis from time to time to obtain different stages of the parasite for future studies of molecular changes in the parasite during differentiation.